Acknowledgments: Supported by the European Research
Council (HYPER project no. 279881), the Strategic International
Research Cooperative Program of the UK Engineering and
Physical Sciences Research Council, and the Japan Science and
Technology Agency. T.M. thanks the funding program for
World-Leading Innovative R&D on Science and Technology
(FIRST Program), Japan, for hybrid solar cell research. We
thank the New Energy and Industrial Technology Development
Organization for support. M.M.L. is grateful for support
from the Simms Bursary granted by Merton College, Oxford.
We thank S. K. Pathak for assistance with x-ray diffraction
measurements and analysis, and A. Abrusci, J. Ball, P. Docampo,
A. Hey, T. Leijtens, N. Noel, and A. Kojima for valuable discussions.
The University of Oxford has filed three patents
related to this work.
Supplementary Materials
www.sciencemag.org/cgi/content/full/science.1228604/DC1
Materials and Methods
Supplementary Text
Figs. S1 to S4

31 May 2012; accepted 7 September 2012Published online 4 October 2012;10.1126/science.1228604Photoinduced Ullmann C–N Coupling:Demonstrating the Viability of aRadical PathwaySidney E. Creutz,1* Kenneth J. Lotito,1* Gregory C. Fu,1,2† Jonas C. Peters1†Carbon–nitrogen (C–N) bond-forming reactions of amines with aryl halides to generatearylamines (anilines), mediated by a stoichiometric copper reagent at elevated temperature (>180°C),were first described by Ullmann in 1903. In the intervening century, this and related C–N bond-formingprocesses have emerged as powerful tools for organic synthesis. Here, we report that UllmannC–N coupling can be photoinduced by using a stoichiometric or a catalytic amount of copper,which enables the reaction to proceed under unusually mild conditions (room temperature oreven –40°C). An array of data are consistent with a single-electron transfer mechanism,representing the most substantial experimental support to date for the viability of this pathwayfor Ullmann C–N couplings.

Arylamines (anilines) are a commonly en- countered subunit in organic compounds and are important in fields ranging from
pharmaceuticals to materials science (1–3). Because many aryl halides and amines are readily
available, coupling these two reactants provides a
particularly attractive, convergent approach to the
synthesis of arylamines. Thus, the discovery by
Ullmann in 1903 that this C–N bond construction
can be accomplished by heating these partners in
the presence of a stoichiometric amount of copper was a landmark achievement (Fig. 1A) (4, 5).
During the past 20 years, there have been numerous advances in C–N coupling reactions, ranging
from the discovery of milder, copper-catalyzed

Ullmann processes to the development of methods based on palladium and other transition metals
(6–11).

Despite the importance of copper-based
Ullmann C–N coupling reactions, understanding
of the mechanism of these processes has evolved
only slowly (6–9, 12). It is believed that Ullmann
couplings generally begin with Cu–N bond formation; however, a variety of pathways for the
subsequent cleavage of the Ar–X bond have been
proposed, including a concerted oxidative addition (13, 14) and a single-electron transfer (SET,
which encompasses halogen-atom transfer) mechanism with radical intermediates (Fig. 1B) (15).
It is likely that different pathways operate under
different conditions.

Currently, there is virtually no direct experi-mental evidence for the viability of an SET mech-anism for Ullmann C–N couplings (12), althoughBuchwald and Houk have recently published acomputational study in support of this pathwayfor certain processes (15, 16). With respect toUllmann C–C bond formation, Kim has observedthat oligothiophenes can be generated upon ultra-violet irradiation (~140 nm) of 2,5-diiodothiopheneon a copper metal surface, presumably via directphotodissociation of the C–I bond (17, 18). Inthis report, we provide an array of experimentaldata for the reaction of aryl halides with well-defined copper(I) amido complex 1, all of whichare consistent with an SET/radical mechanism forUllmann C–N coupling. In addition to furnishingthe strongest evidence to date for the viability ofan SET pathway, this study introduces a photo-induced variant of this powerful transformation.